Formalize the CLI as the first Channel extension (GLAS-WP-0003)

channels/base.py: GatewayInvocation dataclass + Channel ABC
(parse_invocation/render_result), mirroring Rein one layer up -- a
channel turns an external invocation into what run_task_through_rein
needs, and turns its result back into whatever the invocation medium
expects.

channels/cli_channel.py: CLIChannel, a pure refactor of what cli.py did
inline -- argparse Namespace -> GatewayInvocation, result dict ->
json.dumps(..., indent=2). cli.py's run handler is now a thin
construct-parse-invoke-render wrapper.

docs/channel-contract.md documents the pattern and sketches (without
building) a second channel.

Found and fixed a real bug while live-testing this refactor:
ReinAharness's default cli_bin was still "agent-harness", stale from
before HARNESS-WP-0002-T02's rename.

4 new tests (27/27 passing), plus a real live run through
python3 -m glas_harness.cli against rein-aharness/real Claude Code
confirming identical output shape and a real commit.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_cli_channel.py

@@ -0,0 +1,54 @@
+import argparse
+import json
+
+from glas_harness.channels.base import Channel, GatewayInvocation
+from glas_harness.channels.cli_channel import CLIChannel
+
+
+def test_cli_channel_is_a_channel() -> None:
+    assert issubclass(CLIChannel, Channel)
+
+
+def _args(**overrides) -> argparse.Namespace:
+    defaults = dict(
+        sandbox_profile="profile.bwrap-local",
+        repo="/tmp/repo",
+        title="t",
+        description="d",
+        actor="agt",
+        project="glas-harness",
+        no_hub=False,
+    )
+    defaults.update(overrides)
+    return argparse.Namespace(**defaults)
+
+
+def test_parse_invocation_maps_all_fields() -> None:
+    channel = CLIChannel()
+    invocation = channel.parse_invocation(_args())
+
+    assert invocation == GatewayInvocation(
+        sandbox_profile="profile.bwrap-local",
+        repo="/tmp/repo",
+        title="t",
+        description="d",
+        actor="agt",
+        project="glas-harness",
+        report_to_hub=True,
+    )
+
+
+def test_parse_invocation_inverts_no_hub_flag() -> None:
+    channel = CLIChannel()
+    invocation = channel.parse_invocation(_args(no_hub=True))
+    assert invocation.report_to_hub is False
+
+
+def test_render_result_matches_pre_refactor_output() -> None:
+    channel = CLIChannel()
+    result = {"sandbox_id": "sbx1", "tool_ok": True, "summary": {"committed": "True"}}
+
+    rendered = channel.render_result(result)
+
+    assert rendered == json.dumps(result, indent=2)
+    assert json.loads(rendered) == result